Evolution and Internal Structure of the Helvetic Nappes in the Bernese Oberland
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Evolution and internal structure of the Helvetic nappes in the Bernese Oberland Autor(en): Hänni, Reto / Pfiffner, O. Adrian Objekttyp: Article Zeitschrift: Eclogae Geologicae Helvetiae Band (Jahr): 94 (2001) Heft 2 PDF erstellt am: 11.10.2021 Persistenter Link: http://doi.org/10.5169/seals-168886 Nutzungsbedingungen Die ETH-Bibliothek ist Anbieterin der digitalisierten Zeitschriften. Sie besitzt keine Urheberrechte an den Inhalten der Zeitschriften. Die Rechte liegen in der Regel bei den Herausgebern. Die auf der Plattform e-periodica veröffentlichten Dokumente stehen für nicht-kommerzielle Zwecke in Lehre und Forschung sowie für die private Nutzung frei zur Verfügung. Einzelne Dateien oder Ausdrucke aus diesem Angebot können zusammen mit diesen Nutzungsbedingungen und den korrekten Herkunftsbezeichnungen weitergegeben werden. Das Veröffentlichen von Bildern in Print- und Online-Publikationen ist nur mit vorheriger Genehmigung der Rechteinhaber erlaubt. 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Helvetic nappes, western Switzerland ABSTRACT ZUSAMMENFASSUNG During the last century, many detailed field studies have been carried out in Im vergangnenen Jahrhundert ist die lokale Geologie un Berner Oberland in the Bernese Oberland. Data from these studies were combined with new field /ahlreichen Arbeiten detailiert untersucht worden. Darauf aufbauend und dala to restore the Axen and the Drusherg nappes in three cross sections. The kombiniert mit neuen Felduntersuchungen wurden die Drusberg-Decke und criteria for validating retrodeformable cross sections were used to extend the die Axen-Decke in drei Querschnitten abgewickelt. Die Berücksichtigung limits of interpretation. Results show that synsedimentary normal faults played geometrischer Kriterien für abwickelbare Profile erlaubte dabei eine erhebliche a key role in the evolution of the nappes. Retrodeforming the nappes suggests Einschränkung des Interpretationsspielraums in erodierten Deckenteilen. that Cretaceous synsedimentary faults within the Drusberg nappe had reactivated Es stellte sich heraus, dass kretazische synsedimentäre Normalbrüche in Jurassic faults within the Axen nappe. Alpine compression partly reactivated der Deckenkinematik eine Schlüsselrolle spielten. Derartige Brüche findet these faults once again but simultaneous!) developed coniugate reverse man in der Drusberg-Decke: die Deckenabwicklungen lassen vermuten, dass faults: subsequent folding and shearing lead lo Ihe hitherto enigmatic BQrgle- bei ihrer Entstehung jurassische Normalbrüche in der Unterlage, der heutigen Sylere structure. The Schilthorn thrust fault was found to be an important out- Axen-Decke. reaktiviert wurden. Während der alpinen Kompression wurden of-sequence ihrust. causing complex nappe internal structures such as Ihe lec- diese Brüche teilweise reaktiviert und führten in Kombination mit konjugierten tonic isolation of individual Ihrust slices. Ihe Kiental-phase folding of the Rückaufschiebungen und anschliessender Faltung und Scherung zu der Schilthorn fault clearly indicates a polyphase deformation hisiorv of Ihe Drusberg lokal gut bekannten Burgle-Sylere Struktur. Die Schilthorn-Überschiebung and the Axen nappes. isl eine bedeutende out-of-sequence Überschiebung und führte zu komplexen deckeninternen Strukturen in Form tektonisch vollständig isolierter Schuppen. Die Verfaltung der Schilthorn-Überschiebung während der Kiental- Deformationsphase zeugt von der mehrphasigen Verformungsgeschichte der Drusberg- und der Axen-Decke. Introduction The Helvetic zone of Central Switzerland and the Bernese Mesozoic-Cenozoic cover (e.g. Doldenhorn nappe). Further Oberland is composed of two nappe complexes separated by a north, this complex contains allochthonous cover sheets (Sub- major thrust fault with several tens of kilometers of displacement. alpine Flysch and Subalpine Molasse). This major thrust fault is the basal thrust ofthe Helvetic The sediments of the Helvetic zone were deposited on the nappes proper. The Helvetic nappes are characterized by a European margin of Tethys. In the course ofthe Alpine orogeny complex interplay of folds and faults involving Mesozoic- these sediments were thrust and transported to the north Cenozoic cover rocks. The Axen nappe (Fig. 1) contains and northwest. As a general rule, the paleogeographically essentially Jurassic sediments and is overlain by the Drusberg southern Helvetic units experienced a larger amount of nappe, which is built of Cretaceous-Cenozoic strata (Pfiffner displacement than the northern ones, which resulted in the 1993). The units in the footwall of the basal thrust of the development of an antiformal stack on top of the Aar massif in Helvetic nappes are referred to as Infrahelvetic complex. To the the transect ofthe Bernese Oberland (Fig. 1). The Doldenhorn south, this Infrahelvetic complex is a thick-skinned fold-and- nappe contains recumbent folds with a crystalline core forming thrust belt involving pre-Triassic crystalline basement rocks part of to the Aar massif. Across the Lauterbrunnen Valley to (Aar massif) and their autochthonous-parautochthonous the east, the separation between the Mesozoic cover and the 1 Geologisches Institut. Universität Bern. Baltzerstrasse 1. CH-3012 Bern. Switzerland. E-mail: [email protected] 2 now at Geotechnisches Institut AG. Seestrasse 22. 3700 Spiez. Switzerland. E-mail: [email protected] The Helvetic nappes in the Bernese Oberland 161 •—r ~n Section II Violasse Section Mi tri hern Aar massif Doldenhorn nappe Southern Aar massif Gellihorn nappe 5nj Wildhorn nappe «e Axen nappe <*>. Drusberg nappe Unterhasl Subhehetic unn Ultrahlevetie and Penninic nappes Nappe boundary Thrust fault (Ax.-D Section I H t i Normal fault 10 km Lauterbrunnen •: a Q 1 Menta - Q Fig. 1. Tectonic sketch map of the Bernese Oberland. Thick lines are traces of cross sections shown on plate I. crystalline core diminishes and the Doldenhorn nappe passes Helvetic nappes is missing in the Bernese Oberland owing to into the autochthonous-parautochthonous units typical of the erosion. In order to completely reconstruct the eroded and central and eastern Aar massif. subsurface parts of the nappes, results of former investigations From the Kander Valley to the west, the Gellihorn nappe had to be puzzled together. Fieldwork focussed on the upper separates the Wildhorn nappe from the underlying Doldenhorn parts of the Helvetic nappes, which are particularly important nappe. To the east, the Gellihorn nappe quickly diminishes for the tectonic interpretation of the eroded part of the nappes. in volume along strike until its complete disappearance Three balanced sections were constructed along with their east of the Kiental Valley. restored counterparts. The Jurassic and Cretaceous parts of the Wildhorn nappe The aim of this paper is to analyse the importance of ("Stockwerks") exhibit an increasing tectonic independence synsedimentary extensional structures for section retrodeformation going east, resulting in the eventual separation into two and to discuss their effect on the evolution of nappe distinct nappes which can be traced into eastern Switzerland (see internal structures in subsequent Alpine compression. also Pfiffner 1993). the Drusberg nappe (Cretaceous Stockwerk), and the Axen (Jurassic Stockwerk). nappe 2. Cross sections The Jurassic Stockwerk has been shown to contain a number of tight to nearly isoclinal folds (Arbenz 1922: Günzler-Seiffert 2.1. Construction method 1925; Hänni et al. 1997) cut by numerous tear faults (Pilloud 1990) and low angle reverse faults (Schilthorn. Männlichen). Three digitized sections across the study area were balanced The Cretaceous Stockwerk is disharmonically folded above and retrodeformed using suitable computer software such as the Jurassic Stockwerk owing to the thick, incompetent Palfris Geosec 4.0. Despite these powerful tools, section balancing marls separating the two Stockwerks (cf. Pfiffner 1993). As will and retrodeformation requires a lot of iterative work by hand. be shown later the two Stockwerks experienced a displacement Inevitably, the reconstruction of eroded structures and subsurface of up to 10 km by bedding parallel thrusting on this detachment extrapolation demands a certain amount of interpretation. horizon. However, section balancing allows testing of a profile's The Helvetic nappes are overlain by Ultrahelvetic and geometrical correctness by comparing volumes and line-lengths Penninic units of a more internal origin, which outcrop as erosional before and after deformation. In addition, a retrodeformable relics or "klippen" (Fig. 1). cross section not only has to be geometrically correct but it As a rough estimate,